1. Field of the Invention
The invention concerns a process for the liquefaction of coal, in which the ground up coal is mixed with grinding oil into a paste and is then liquefied with increased pressure and temperature in a liquefaction zone in the presence of hydrogen and, possibly, a catalyst.
2. Prior Art
A process for the liquefaction of coal is already known, in which the coal to be processed is dried and finely ground., and is mixed with grinding oil. The produced coal paste is pumped to reaction pressure, is then heated by heat exchange with a part of the reaction products and, following this, it is heated to the start-up temperature of the liquefaction reaction under the addition of external heat. Finally, it is liquefied in a reaction area in the presence of hydrogen and one or more adequate catalysts. The product fraction exiting the reaction area is decomposed into a steamy overhead fraction consisting of gases, reaction water and distillation oils, as well as into a bottom fraction containing solid matter from not dissociated carbon, ashes, catalyst particles, other high molecular substances difficult to hydrogenate, such as, above all, asphaltenes and fuel oils.
While the overhead fraction is cooled by the heat exchange with the coal paste and is extracted from the unit, the distillates still present from the bottom fraction are separated and are used as grinding oil for the fresh coal.
The disadvantages of this process reside in the high consumption of energy, as well as in the considerable complexity of the devices.
Therefore, the entire amount of coal to be processed must be dried in a special drier with the use of external heat. In the event of a normal water content of approximately 10%, such as in the case of bituminous coal as a raw product, to dry a ton of coal, an energy of approximately 1 Gcal is necessary. In addition, the coal must be ground to a kernel size of &lt;0.1 mm. In this known process, the heat exchange process to heat the coal paste is very difficult. On one hand, due to the viscosity of the coal paste, it is very difficult to obtain an even coating of the heat exchanger surfaces. On the other hand, the continued heating of the coal paste in the preheater creates difficulties, since, due to the high temperatures already present, the coal suspended in the coal paste will swell considerably. This leads to a further increase in viscosity, so that lastly, a transport of the coal paste through the preheater is only possible with a strong materials drive, connected with a pulsating passage of the coal paste through it. At this point, sudden pressure increases of up to 10 bar can be generated.
With this known procedure, additionally, the desired yield of distillate oils greatly depends on the hydrogenation conditions (such as pressure and temperature) in the reaction area. Basically, a satisfactory yield of distillate oils of above 50% (based on the type of coal employed) can only be achieved with very acute hydrogenation conditions, with temperatures in the range of about 480.degree. C., and with pressures above 300 bar. It is evident that such conditions, in a large technical unit, are connected with high investment and operation costs, which in turn have a considerable effect on the economy of the process.